It is known that, during a flight, the crew of an aircraft, in particular of a civilian transport airplane, regularly has to fly this aircraft at an altitude that is different from the altitude defined in the flight plan. This generally occurs following an instruction originating from an air traffic controller. The crew must then make the airplane rejoin the flight plan when the controller gives the appropriate authorization, in order to continue the flight and once again follow the vertical profile defined by this flight plan.
Currently, to rejoin (or pick up) such a vertical profile, the crew has several piloting (or vertical guidance) modes. It mainly uses a descent mode, a vertical speed mode or a gradient mode.
The descent mode offers a vertical guidance enabling the crew to follow the vertical profile calculated by a Flight Management System (FMS) and defined in the flight plan. When the crew of the airplane flies at an altitude that is different to that of the vertical profile, it has the option to engage this descent mode which, in this case, completely manages the picking-up and servo-controlling of the airplane to the vertical profile. Once the mode is engaged, the crew is no longer in control of how the airplane rejoins the vertical profile. The crew totally delegates the management of the pick-up process to the automatic pilot and does not have the option to control the path of the airplane.
In the context of the present invention, the expression “picking up a vertical profile” should be understood to mean the process of approaching this vertical profile and arriving at this vertical profile.
Moreover, it is known that predictions relating to the path followed by the airplane, made by the flight management system, are calculated assuming that the crew in this case handles the descent with half of the air brakes extended. However, in reality, it is not uncommon for the crew to be in a different configuration, which the flight management system does not take into account. Consequently, the path predictions of the flight management system often differ from the actual path of the airplane. Thus, it may be that the crew does not have information concerning the path that the airplane will follow or concerning the vertical speed that the aircraft will assume, with such a descent mode.
The vertical speed and gradient modes are piloting (or vertical guidance) modes which enable the crew to choose a vertical pick-up path, by respectively selecting a vertical speed or a gradient which is defined by a climb or descent angle of FPA (Flight Path Angle) type. When one of these two modes is engaged, and when the path selected by the crew passes through an altitude selected on a Flight Control Unit (FCU), the airplane follows the path to this altitude and then levels off. These modes thus make it possible to control the path of the airplane, but they do not make it possible to pick up the vertical profile. In fact, they do not take this vertical profile into account at all.
Consequently, a crew which is located at an altitude that is different from the altitude defined in the flight plan and that wants to control the pick-up path has no solution enabling it to do so easily.
Indeed, as indicated previously, when the aircraft is not flying along the vertical profile, the crew has no direct means for controlling the picking-up of the vertical profile and servo-controlling the aircraft thereto. Since the picking-up of the vertical profile with the above-mentioned descent mode is quite vague to the crew, it often happens that the crew wants to control the picking up of the profile in order to control the point at which the vertical profile is picked up (that is, the moment when the aircraft actually rejoins the vertical profile).
Furthermore, a use of the vertical speed mode to approach the vertical profile and of the descent mode for definitively picking up and servo-controlling to said vertical profile is not a satisfactory solution. In practice, the moment of engagement of the descent mode depends only on the assessment of the crew. Now, a late engagement of this descent mode can possibly affect the safety of the airplane. For example, in the case where an altitude constraint is defined in the flight plan in order to ensure a sufficient distance between the airplane and an obstacle (notably the relief), the failure to comply with the constraint can lead the airplane to approach the obstacle, all the more so since, under the vertical profile, the descent mode makes the airplane join this vertical profile with a high vertical speed, for example of around 1000 feet/minute.
Consequently, the crew of an airplane has no simple and safe means:
for controlling the path for picking up (or rejoining) a vertical profile;
for effectively picking up this vertical profile; and
for servo-controlling thereto,
in all the cases where the airplane is not located on said vertical profile.
Moreover, document FR-2 870 610 discloses a method and a device for automatically determining a path for picking up a flight path for an aircraft.